TMZ broke the story ... Shawn was riding on a train en route to the Minneapolis-St. Paul International Airport back in September ... when an allegedly drunk man started talking smack to one of the passengers. Daivari says the man was shouting racial slurs and threatened to kill one of the people on the train.

That's when Daivari decided to take action -- the video shows Daivari calmly removing his jacket and sunglasses .... and then approaching the man from behind where he applied a ferocious rear naked choke.

As Daivari applied more pressure, the man fell to the ground -- eventually losing consciousness and urinating on himself.

Moments later, Daivari and another passenger grabbed the man and threw him off the train on to a platform -- with Daivari delivering a few kicks for good measure.

Daivari had previously told us, "I wasn't going to wait any longer to see if a cop showed up."